Explained by Warren Day
On the choice of deposit models
In Wyoming, the Proterozoic rocks exposed in Precambrian uplifts contain several small massive sulfide deposits and occurrences that have characteristics similar to kuroko massive sulfide deposits. These deposits generally form near the top of bimodal submarine volcanic sequences in island-arc-related extensional basins near centers of felsic volcanism and range from Archean to Cenozoic in age. The upper parts of the deposits are stratiform and contain sulfide minerals of copper, zinc, and iron; there is commonly an underlying zone of stockwork, stringers, and veins rich in copper and iron sulfides. The descriptive model of Singer (1986) was used for the assessment.
On the delineation of permissive tracts
Several Precambrian terranes in Wyoming, north of the Nash Fork-Mullen suture zone, have potential for kuroko-type volcanic-hosted massive sulfide deposits, which are important sources of copper and zinc and commonly have byproduct gold and silver. The criteria for delineation of the permissive tract include: (1) submarine volcanic host rocks of felsic composition; (2) island-arc tectonic setting with local extensional deformation at the time of mineralization; and, (3) exhalative horizons within the supracrustal sequence associated with the volcanic rocks (for example, iron-formations or cherty horizons) that indicate the evidence for submarine hydrothermal hot-spring activity during volcanism. The team outlined 14 areas that meet the criteria outlined above, and, together, they constitute the permissive tract. The criteria used for this assessment followed those outlined by Eckstrand (1984) and Singer (1986) for regional evaluations of Precambrian terranes similar to those of Wyoming.
Important examples of this type of deposit
Although prospects of this type are known in several locations, few had reported production.
On the numerical estimates made
The assessment team wishes to acknowledge the helpful suggestions of W. D. Hausel of the Geological Survey of Wyoming. These estimates were tempered with the fact that there has been minimal development of massive sulfide deposits of this type in Wyoming. The area thought to have highest probability was the Hartville uplift region, with an estimated 8 percent chance of hosting such a deposit. The volcanic and sedimentary rocks of the South Pass-Atlantic City area was estimated to have a 5 percent chance to host a volcanic-hosted massive sulfide deposit, based on the presence of the large banded iron-formation deposit at South Pass, Wyoming. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 3, and 4 deposits consistent with the grade and tonnage model of Singer and Mosier (1986).
Eckstrand, O.R., Canadian mineral deposit types—A geological synopsis: Geological Survey of Canada Economic Geology Report 36, 86 p.
Singer, D.A., 1986, Descriptive model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 189-190.
Singer, D.A., and Mosier, D.L., 1986, Grade and tonnage model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 190-197.